Designing a ZigBee Network for Signal Perception

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Abstract
Sensor networks based on the IEEE 802.15.4 standard are simple and low-cost. The IEEE 802.15.4 standard provides two modes of connections: beacon enabled mode and non-beacon enabled mode. The beacon enabled mode can offer transmission determinism thanks to Guaranteed Time Slots (GTSs). The non-beacon enabled mode does not offer any guarantee on traffic determinism. Contrary to the non-beacon enabled mode, the beacon enabled mode does not allow us to form mesh topology in order to interconnect several beacon networks. The current solution to interconnect beacon networks is to use a mesh network made of non-beacon devices that can compromise the determinism of GTSs. In this paper, the notion of beacon-aware device is introduced. A beacon aware device acts as an interface between an in range beacon network and a mesh network. Unlike a non-beacon device, a beacon aware device gives priority to the in range beacon traffic, in order to avoid any perturbations of the beacon traffic. This priority is obtained with a modification of the slotted CSMA/CA algorithm, implemented on the beacon aware device. The beacon aware uses a modified CSMA/CA to send traffics issued from a mesh network having for destination in range beacon devices. The beacon aware device uses the simple CSMA/CA algorithm (unslotted version) otherwise.
Keywords: ZigBee, IEEE 802.15.4, Beacon, Non-beacon, Signal perception, MAC, CSMA/CA.
1. Introduction
ZigBee standard from the ZigBee Alliance provides a protocol for sensor networks. It provides mechanisms for network establishment, device communication and packets routing. Networks implementing this standard are low energy consumption, self-organized and can operate on one of the three frequency bands: 2.4 GHz, 915 MHz and 816 MHz. In this paper we are interested in the 2.4 GHz band. However, the solution presented can be implemented in the two other bands by taking into consideration the specific parameters of each band. The ZigBee standard is based on the IEEE 802.15.4 Designing a ZigBee Network for Signal Perception 265 standard for Medium Access Control (MAC sub-layer) and for wireless transmissions and receptions (Physical layer). Two topologies are available in the IEEE 802.15.4: mesh topology and star topology (see Section 2.1 {topologies}). The MAC sub-layer allows two modes for transmitting and receiving data: beacon enabled mode and non-beacon enabled mode. The first can guarantee transmission determinism within Guaranteed Time Slots (GTSs), but needs synchronization between all devices which form the beacon enabled network. The latter does not give any traffic guarantee and does not need synchronization between devices (see subsection 2.2 {non beacon network}). In the current ZigBee specification, the interconnection of beacon networks is done by using a mesh network formed by nonbeacon devices. However, non-beacon devices do not take into consideration the beacon traffic and can perturb the beacon devices transmission causing the loss of the transmission determinism in the GTS slots (see subsection 2.3 {Beacon enabled network}). The main contribution of this paper is the introduction of a new device named Beacon Aware device in the IEEE 802.15.4 protocol in order to interconnect beacon enabled networks to the mesh network made of non-beacon devices. A beacon aware device is a expansion of a non-beacon device which takes into consideration the beacon traffic in order to preserve the beacon traffic from perturbations and thus preserving the transmission determinism. In addition to some minimal changes made on the IEEE 802.15.4 protocol, the principle
Advantages of a beacon aware device are:
• Avoiding all perturbations of the beacon traffic thanks to a priority mechanism (see section 4 (the beacon aware device)) that gives priority to the beacon traffic for channel access.
• Acting as an interface between the beacon network and the non-beacon network by routing packets from and to the beacon network. The beacon aware device respects the beacon's super frame specifications. Thus, the beacon aware device does not transmit during the inactive period in order to avoid packets loss, unless if the destination node is in a different beacon network.
• Devices operating in beacon aware mode are: self-organized, define their own energy consumption policy, do not transmit periodically beacon frames, use CSMA/CA in its slotted version (when interacting with beacon devices) and unslotted version(when interacting with non-beacon devices) and perform routing tasks (see Section 4 {the beacon aware device}).
This work was mainly inspired from papers exposing solutions for increasing the transmission determinism in beacon enabled mode. It also responds to the need of interconnecting distant beacon networks. The focus on the GTS allocation mechanism to increase the number of devices using the GTSs. In [7] the authors introduce an Implicit GTS Allocation Mechanism (i-game).
This mechanism allows the increase of the number of nodes using GTSs in order to increase the traffic determinism without increasing the number of GTS in the coordinator Super Frame. Beacon nodes can share the same GTS if they satisfy some conditions on traffic specifications and delay requirements. A Management algorithm is implemented on the PAN coordinator. This algorithm determines whether the new GTS request will be accepted or not and if this node shall share an existing GTS or allocate a new GTS for it., the authors introduce a new MAC protocol based on the ZigBee/IEEE 802.15.4 standard. This protocol is designed for time sensitive sensor networks requiring a deterministic medium access. It introduces new functions for the PAN coordinator which must guarantee GBS (Guaranteed Beacon Slot) for each beacon coordinator present in the network and validate the allocation of GTSs of its child beacon coordinators. The PAN Coordinator must have a global view of the network. Thus, before allocating a GTS to an associated node, the beacon coordinator consults the PAN coordinator. If this allocation will cause perturbations on a previously allocated GTS (for a node associated with another beacon coordinator), then the allocation request should be rejected. The authors of this paper [8] also introduce a new way of allocating GTS. An allocated GTS does not necessarily appear on each super frame, the node requesting a GTS can specify the frequency of the GTSs. Hence, 266 I. Benakila, S. Femmam and L. George several traffics can cohabit in the same beacon network. The beacon aware mode exposed in this paper differs from these quoted works in two main points.
First, we do not consider that we have only one beacon network. We suppose that beacon networks are interconnected with a mesh network.
Second, our work always gives the priority to the beacon transmissions. It enables us to grant the determinism of beacon transmissions within GTSs and at the same time allows the interaction of several beacon networks. Notice that, these related approaches are compatible with our work. In fact, our work does not make any changes on the beacon enabled mode. We introduce a new device able to communicate with beacon devices and non-beacon devices.
In section 2, we recall some of the IEEE 802.15.4 principles and detail the beacon mode and the non-beacon mode. In section 3, we introduce the context of our study, define the problem and the constraints that the solution must respect. Section 4 presents the core of the solution where we introduce the beacon aware device and its main mechanisms. In section 5, we provide some results of simulation made of a ZigBee network with beacon aware devices based on the network simulator (NS2) tool. We mainly focus on the beacon traffic preservation property. Finally, we conclude.
2. IEEE 802.15.4 Overview
The IEEE 802.15.4 standard is a suitable protocol for low rate wireless networks. The ZigBee protocol for sensor networks is based on the IEEE 802.15.4 standard to define the medium access (MAC) sublayer and the physical sub-layer. There are two kinds of ZigBee devices: full function devices (FFD) and reduced function devices (RFD).
• A FFD implements all the standard's functions. A device of this kind may operate as a coordinator, as a router or as a simple device. A coordinator is the device responsible for starting and maintaining the network.
• A RFD device implements only a part of the functions defined in the standard. For example, an RFD cannot start a network, cannot route packets, etc... This kind of devices can only operate as simple devices associated to a coordinator. Since, a beacon aware device should be able to perform routing task and accept device association, it must be implemented as an FFD device.